Pencil beam slot antenna



March l5, 1960 K. c. KELLY 2,929,064

v PENCIL BEAM sLoT ANTENNA Filed Aug. 2, 1957 2 Sheets-Sheet 1 March l5,1960 K. c. KELLY 2,929,064

PENCIL BEAM sLo'r ANTENNA v Filed Aug. 2, 1957 2 Sheets-Sheet 2 In, ai

l'wwnm @awww-EE. Wg

PENCIL BEAM SLQT .Kenneth C. Kelly, Los Angeles, Calif., assigner to`Hughes Aircraft Company,v Culver City,V Calif., a` corporation of.Delaware Application August 2, 195:7,.SrialNo'. 675,909-

Claims. (Cl. 343-771) This invention relates to wave energy' radiatorsand more particularly' to' anantenna for; radiating a plane polarizedpencil beam.

Electromagnetic energy beams of circular cross section, which arecommonly referred toas pencil beams; are extensively utilizedY for' the'location or tracking of reliecting objects, and for polnt-to-pointmicrowave communication systems; Such beams maybe plane or circularlypolar'- iaed. Heretofore, plane polarized pencil beams have beenobtained,.for example, by combining a plane polar'- ized electromagneticwavepoint source with a beam shaping re'ector. 'Such reflectors usuallyoccupy a large volume',A however, andi therefore'` are notitoovdesirable in systems where space is. at a' premium. p .Another methodwhich; has' beenernp'loyed forl the production of. pla-ne.- polarizedpencil beamsv utilizes spatial arrays of planel polarized sources'y suchas a plurality of dipoles or slots.. Suchl systems may in someapplications haver the advantage of' eliminationv of the space wastingreflector- However, these' arrays are usually diiiicultE to fabricate:and, further, often. require a complicated' feed systemimposing'complex' electrical requirements.

Radiatin'g horns have also been utilized for the productionfo planepolarized' pencil beams. While such horns haveb avoided the' complicatedfeed system attendant in spatial arraysg. they have'. in mauy'casesrequired a great depth in the direction of the radiating beam and havebeen found unsuitable when. space' is'at a premium. The same limitationsare usually present' insurfacer Wave antennas. whichf are also capableof producing plane polarized pencil. beams.. As' is wellV known, asurface wave antenna suitableY for' the production of ay plane polarizedpencil beam` requires very great length' to sufiiciently narrow thebeam. in. a plane perpendicularr tothe surface.

Recent' attempts to develop' a'y radiator to produce a plane'polarized.beam have utiiized.' annular slots. excited by a structure' setting upeither radial or; circumferential electricelds in the slots. Suchsystems overcomethe objectionable. depth of the' antenna and the highcost atg tendant toits manufacture. The-beams so produced are; however,elliptical in crosssection. along thev radiator axis and` the side lobelevel varies Withthe plane of observation. Furthermore, there aredirections in space in which prominent cross polarization exists.

It is therefore an object of this invention to provide an antenna forradiating a plane polarized pencil' beam wi'i'icli is excited by asimpie feed structure' and' which exhibits only` a small amount' ofcross polarization.

It is also an object of this' invention to provide a plane polarizedpencil beam radiator which occupies a'- small vol'umeandha's a' smalldepth.

It is a further object of this invention to provide a' plane polarizedpencil beam antenna which isy simple to manufacture and which has a lowside lobe'level.

It is another object of this invention to provide an improved planepolarized pencil beam radiator which is simple in construction,reiiablev in operation, small in volume,Y light in weight and provides.ease. of aperture, control to facilitate beam. design.

32,064 Patented Mar. i5, i960 lnl accordance with one embodiment ofthepencil beam antenna of this invention, a conductive plate isperforated with a large number.' ot"` slot-type radiation aperturesdis"- posed alongv a number of concentric semi-circles. Currents areinduced into' the plate by electromagnetic waves in such a'mannery thatthe gap produced by any radiation aperture intercepts a portion of thecurrent and' causes excitation of that aperture. TheV directions ofthecurrents induced into the plate-and the locations of the aper# turesinthe plate are selected in such a manner that the electromagnetic wavesradiated into space by each aperture'are all substantially planepolarized along a'- common plane of polarization.

Plane polarizationv along a common plane` is obtained in th'eselected'embodiments of this invention by inducing a set' ofcircumferential bands of radial or of`ci1cu'm'- t'erential current' intoth'esurface of the conductive-plate. Thel slot-'type radiation aperturesare located in such a way' as' to intercept the induced currentsubstantially in the same spatial direction and are inclined to radiatea component along the common plane of polarization.

' The novel features which are believed to be characteristie oftheinvention, both as to its organization and method? offoperation',together withl further' objects andl advantagesfthereof, will be betterunderstood from the followingVL description considered in' connectionwith" the accompanying drawings in which two` embodiments 'of thisinvention are illustrated by way of* example. It' is t'o bey expressly4runderstood", however, that the drawings are for the purpose ofillustration and? description only; and are' not intended" a's adefinition' of the limits of' thev invention.

Figs; 1T and 3v are plan' views of differentI embodiments i of the"radiators; of this invention;

Figs; 2 and lareJ cross-sectional views taken respectively along thelines 2"-2'l and' il-4' of' theL radiators of Figs. 1' and' 3i; and:

Figs. Sand 6arevillust'r-ative" radiation patterns of tli'e' radiatoroifFig. 3 measured through a pl'an'e perpendicular to the aperture planeand containing the radiation' axis'i Ref'erringfnow'to the drawingsg'inwhich like reference characters designate like'A parts, there isshowni'rrFig: 1i a planar' conductivev plate'v if perforated with` alargeI nume ber of radiation apertures lf2wliicli'havef the shapeofashort thin slot'. Y The radiation' apertures i2 are disposed within theplate along a plurality of semi-circles of which four are shownA and"=designated by the reference characters concentric-and have acommon'geometrical center. at`a point 22. ."lhe-point. ZZli'es on aradiator-.axis shown. ih` Fig.. 2` whichisfperpendicular. to plate 10'and pierces; plate 10 at the center pointfZZ.

The-semi-circlest14,15,18 and Ztl?l differ. in. radius. by one-half of.a. working wavelength'ori an odd vmultiple thereof. Thetermworkingwavelength? will bei'deiine'd in connection with theexplanation of the` operationfof this invention. The semi-circles 14,16,1S1 and1V 2t)E are also disposed', in the order 'of'.increasing radialdimensions alternately 'to the' right' and to' the' left of arst'diam'etrica'l plane 26whi`ch first ldiame'trical plane contains' theradiation axis 2i`('shownY in Fiv. 2^).

The slots 12 are each inclined with respect to aradialiZ line 28`vdrawn' from the center point 22 to the' centerpoint 30 of each slot.This inclination' depends upon'an' angle 0 which the radialline 28makes. with the iirst diametricalf plane 26. Angle 0 is positive whenmeasured in an anticlockwise direction about. center 22. from the.planezol The inclination of aslotis measured by anangl'e, isV positivewhen. measured. in an anti-clockwise direction about/the slot center.Sti fromy the radial` lineZia",`-

For the purpose of describing the slot inclination without equivocation,the conductive plate 10 isdivided into four quadrants by a seconddiametrical plane 32 perpendicular to the first diametrical plane 26 andintersecting the first diametrical plane along the radiator axis 24. Thefirst quadrant is'dened by sweeping the radial line 2S through the angle0 between zero and 90 degrees. Similarly, the second, third and fourthquadrants are defined respectively for values of between 90 degrees to180 degrees, 180 degrees to 270 degrees,'and 270 degrees to 360 degrees.The inclination tlf of the lslots 12 with the radial line 28 in thefirst and the third quadrant is preferably plus 45 degrees and in thesecond and fourth quadrant is preferably minus 45 degrees. For optimumcondition of operation the inclination tlf should be as close aspossible to 45 degrees. t

The slots are approximately one-half of a free space directed alongradii and the radial sense may be either inward or outward.

If the radial waveguide 34 is excited and the circumferential band ofcircumferential currents are induced into the plate 10, the center ofthe current band is either stationary for standing wave fields orexpanding for travelling wave elds. For standing wave fields, the centerof a current band may be made to coincide with the semicircle 14. Sincethe radial separation between the semicircles 14 and 16 is equal toone-half of a working wavelength or odd multiples thereof and the radialdistance between thev center of the current bands is likewise onehalf ofa working wavelength, the semi-circle 16 also Wavelength in length andone-twentieth of a free space wavelength in width. The exact length andthe number of the slots on eachv semi-circle depends on the fraction lofthe total power to be radiated by the slots lying on a givensemi-circle. The fraction is determined, for each `semi-circle by itsrelative position from the center point 22. This is a matter of designand serves to control the side lobe level of the energy radiated fromthe radiator.

Referring now to Fig. 2, the conductive plate 10 is coupled to radialwaveguide 34. Radial wayeguide 34 is utilized for the purpose ofinducing currents'into plate 10 for exciting the slots 12. The radialwaveguide 34 includes a lower conductive plate 36 and a cylindrical wall38 substantially concentric with the radiator axis 24. As will becomemore evident from a description of the pperation of the radiator of thisinvention, the wall 38 terminating the radial waveguide 34 may be of aconductive materiai when it is desired to excite the slots 12 bystanding wave fields. The slots may also be excited by travelling wavefields and for this embodiment the wall 38 may be of an absorptivematerial which will provide a termination of the radial waveguide 34`suitable for maintaining travelling wave fields. In other words, theconductive plate 10 may be excited by travelling wave fields or bystanding wave fields and will, in either case, produce a plane polarizedpencil beam.

The radial waveguide 34 may be excited by a feed means such as thecircular waveguide 40 which is dimensioned for propagation for theTEM-mode of the circular waveguide. The circular waveguide 40 is coupledto the radial waveguide 34 and disposed symmetrical with the radiationaxis 24.

The operation of the radiator shown in Figs. l and 2, will now beexplained. The circular waveguide 40 being dimensioned for propagationof the TEM-mode of the circular waveguide will excite the Hofmode of theradial waveguide between the conductive plate 10 and the con-A ductiveplate 36 since the spacing of these plates-is over one-half of a freespace wavelength. If the wall 38 is made of a conductive materialstanding wavefields will be set up. If the wall 38 is made of anabsorptive material, travelling wave fields will be set up in the radialwaveguide 34.

The waves of the Hol-mode of the radial waveguide,

. as is well known, induce circumferential bands of cir.

cumferential current into the conductive plate 10. The termcircumferential band of circumferential current is defined as an annularregion of surface current in plate 10 in which all current lines fiow inthe same rotational sense which may be clockwise or counterclockwise.The intensities of the current lines in a band depends on the distancefrom center 22 at the instant under discussion. Each band starts andends with zero intensity lines if the observer were moving in the radialdirection with time standing still. The term circumferential bauds ofradial coincides with the center of a current band. However, thedirection of the circumferential current along semicircle 16 is oppositeto that of the current along the semicircle 14. The term workingwavelengt here used refers to the asymptotic value of the wavelength ofthe Hankel function which describes the radial dependence of waves in aradial waveguide.

The circumferential currents excite the short thin slots 12 by settingup an electric field across the narrow dimension thereof. The vectorsdesignated by the reference character E (Fig, l) represent aninstantaneous condition for the direction and magnitude of the electricfield across a selected few of the slots 12. As can. be seen from Fig.-l when all the vectors E are added up with one another, a planepolarized wave having a plane of polarization parallel to the dimetricalplane 26 is obtained;

Along the radiator axis 24 this polarization is substantially withoutany cross polarization. At a certain point not in the first or seconddiametrical plane a certain amount .of

cross polarization exists. The cross polarization which is defined asradiated wave energy having a plane of polarization perpendicular to thefirst diametrical plane has a maximum intensity of approximately 12 dbbelow the main beam for one pair of semi-circles. This presents a markedimprovement (about 4 db) over the annular slot .type radiator previouslymentioned. This cross polarlzation may be further suppressed by addingmore seml-crrcles. Such suppression is not obtainable by addg ingannular rings to the annular slots antenna.

For traveling wave fields, the operation of the radiator ofthisinvention is likewise described by the foregoing explanation.Instead of regarding the center of the current bands as fixed in space,the instantaneous position of the center of a current band isconsidered. At a. given instant of time, the operation of the radiatoris exactly the same. The difference over a time interval correspondingto a cycle at the operating frequency is that when standing waves areused to excite the radial waveguide 34, the induced current band remainsstationary and the current amplitude goes through a complete 46, 48 and50. The radial dimensions and the relative current to be used inconjunction with Figs. 3 and 4 are similarly described in that there areannular regions of locations of the semi'circles 44, 46, 48 and 50 aresimilar to those of the semi-circles 14, 16, 18 and 20 of Fig. 1. Theyare concentric about a point SZVthrough which the radiator axis 54passes as shown in Fig. 4.

The essential difference between the conductive plate 10 of Fig. l 'andthe conductive plate 40 of Fig. 3 is that the semi-circles of Fig. 3 aredisposed about the second diametrical plane 56 which is perpendicular tothe first diametrical-plane 58. The first diametrical plane 58 isdefined as above as the direction of the plane of polarization of theWaves radiated by the radiator.

The same nomenclature and geometry may be adopted to designate theinclination of the slots 42. Therefore, the angle and the angle rb arethe same as defined in connection with Fig. 1 together with thedesignation of the four quadrants. The inclination of the slots 42 istherefore plus 45 degrees in t'ne first and the third quadrant and minus45 degrees in the second and fourth quadrant.

It is therefore seen that the inclination ofthe slots in the radiator ofFig. 3 is identical .to the inclination ip of the slots in the radiatorof Fig. l. However, the semicircles upon which the slots are located areacross different diametrical planes. The semi-circles of the Vradiatorof Fig. 3 are located on opposite sides of a diametrical planeperpendicular to the direction of the plane of polarization whereas thesemi-circles of the radiator of Fig. l are located on opposite sides ofa diametrical plane parallel and coinciding with the plane ofpolarization.

Referring to Fig. 4, there is shown a preferred embodiment of anexciting means for the conductive plate 40. The conductive plate 40 iscoupled to a conical waveguide 69 which comprises a conical surface ofrevolution 62 oppositely disposed to and symmetric with plate 4t). Theconical waveguide 60 is terminated. The termination may comprise aconductive circular wall 64 having an absorptive lining 66 formaintaining travelling wave fields with the conical waveguide 60. Theabsorptive lining 66may be dispensed with if standing wave fieldsaredesired for exciting the slots 42. As indicated previously, theconductive plate 40 may be excited with either standing waves ortravelling waves, both methods of excitation will provide a planepolarized wave from the radiator of Fig. 4.

The conical waveguide 6) may be excited by a feed means such as acoaxial waveguide 68 having an outer conductor 70 and a center conductor72. The outer conductor is coupled to the conical surface of revolutionand the inner conductor is connected to the plate 40. The coaxialwaveguide 68 is adapted to be excited in the TEM-mode of a coaxialwaveguide and will excite the fundamental conical waveguide mode betweenthe plate 40 and the conical surface of revolution 62.

The operation of the radiator of Fig. 4 is in many respects similar tothat of the radiator of Fig. 2. The major difference lies in directionof the currents induced into plate 40 by the fundamental conicalwaveguide mode. The fundamental mode induces circumferential bands ofradial currents so that the direction of current flow in the plate 40 isradially inward or outward. The inclined slots 42 intercept, as before,a component of the current which is perpendicular to the long dimensionof the slots. This intercepted component gives rise to an electric fieldacross each of the slots 42 which are designated by vector E for a fewof the slots.

The radial dimension of the semi-circles 44 and 46 differ by one-half ofa free space wavelength since the fundamental mode is a TEM-mode. In thecase of standing wave field excitation, the center of two adjacentcurrent bands coincide with semicircles 44 and 46. The radial directionof the current in the two bands is reversed and proper slot excitationis obtained giving rise to a plane polarized wave having a plane ofpolarization parallel to the first diametrical plane 58.

The conical transmission line shown in Fig. 4 may be replaced by aradial waveguide or any symmetrical waveguide for that matter, as longas the distance between adjacent semi-circles is kept at one-half of aworking wavelength at the particular radial dimension, and the mode usedexcites radial currents.

Figs. and 6 respectively show the radiation pattern obtained with theradiator of Fig. 4. Fig. 5 shows the radiation pattern 80 which isobtained with a radiator having but two semi-circles as shownschematically by the two semi-circles 81 and 82. Fig. 6 shows theradiation pattern 84 of a radiator having four semi-circles as shown bythe semi-circles 85, 86, 87 and 88.

A comparison between the pattern St) and the pattern 84 c shows thegreater the number of semi-circles, the sharper is the radiation patternalong lthe radiator axis 54 which corresponds to zero degrees of thepattern plot. It further shows that the side lobe level is alsodecreased substantially by increasing the number of semi-circles..

There has been described a radiator which may b e utilized for radiatinga plane polarized pencil beam which v essentially comprises a conductiveplate perforated with` a large number of thin short slots. Means areprovided for exciting the conductive plate by circumferential bands ofeither radial or circumferential current. The thinshort radiation slotsare disposed within the conductive plate in such a manner as to increasethe field contributions to 'the desired plane of polarization and reducethe contributions perpendicular thereto while causing an improvement 'ofthe shape of the main beam.

What is claimed as new is: l

1. A radiator comprising a conductive plate having la plurality ofradiating apertures therethrough, all of the apertures through saidplate forming a pattern that is asymmetrical about a line extendingdiametrically across vsaid plate, said pattern including a first row ofapertures and a second row of apertures, all of the apertures in said`first row being disposed a uniform distance from a comcenter, all ofthe apertures disposed said common distance from said center beingdisposed on the opposite side .of said line, said common distance andsaid uniform distance differing from each other by some predeterminedamount, all of the radiating apertures on one side of said line having adistance from said common center differing from the distance from saidcenter of all of the radiating apertures on the other side of said line,and means coupled to said plate for inducing a predetermined currenttherein for exciting said apertures whereby electromagnetic energy willbe radiated therefrom.

2. A radiator comprising a conductive plate having a plurality ofradiating apertures therethrough, all of the apertures through saidplate forming a pattern that is asymmetric about a line extendingdiametrically across said plate, said pattern including a first group ofapertures and a second group of apertures, all of the apertures in saidfirst group forming semicircular rows that are disposed entirely oneside of said line and are concentric about a point located on said line,all of the apertures in said second group forming semicircular rows thatare disposed entirely on the opposite side of said line and areconcentric about said point, the geometric centers of all of theapertures in any given row being disposed some predetermined radius fromsaid point, all of the apen` tures having their geometric centersdisposed at a given radius from said center being disposed on the sameside of said line, the radius of any given row differing from the radiusof any given row in the other group by a predetermined amountcorresponding to an odd integer multiple of one-half a wavelength of theradiated energy, and means coupled to said plate for inducing apredeter-A mined current therein for exciting said apertures wherebyelectromagnetic energy will be radiated therefrom.

3. A radiator for radiating electromagnetic energy along a radiationaxis, said radiator comprising a conductive plate having a plurality ofelongated radiating `slots therethrough, all of the slots through saidplate forming a pattern that is asymmetrical about a line extendingdiametrically across said plate, said pattern including a first row ofslots disposed on one side of said line and a second row of slotsdisposed on the other side of said line, all of the slots in said firstrow having the geometric centers thereof disposed a uniform distancefrom acommon point located on said line, all of the slots having thegeometric centers thereof disposed said uniform distance from said pointbeing disposed on the side of said line containing said first row, allof the slots in said second vrow being 7 disposed a common distance fromsaid center, all of the slots disposed said common distance from saidcenter being disposed entirely on the side of said line containing saidsecond row, said common distance and said uniform distance differingfrom each other by an odd integer multiple of one-half a wavelength ofthe radiated energy, all of the radiating slots on one side of said linehaving a distance from said common point differing from the distancefrom said common point differing from the distance from said center ofall radiating slots on the other side of said line, a second conductiveplate spaced from said iirst plate so as to form a circular symmetricalwaveguide, and means coupled to said waveguide for exciting a singlewave energy mode therewithin that will excite said slots wherebyelectromagnetic energy will be radiated therefrom along said axis.

4. A radiator for radiating a beam of electromagnetic energy along aradiation axis, said radiator comprising a iirst conductive platedisposed normal to said radiation axis and having a plurality ofelongated radiating slots therethrough, all of the slots through saidplate forming a pattern that is asymmetrical about a line extendingdiametrically across said plate and intersecting said radiation axis,said pattern including a iirst row of slots disposed on one side of saidline and a second row of slots disposed on the other side of said line,all of the slots in said first row having the geometric centers thereofdisposed a uniform distance from the intersection of said axis and saidline, all of the slots having the centers thereof disposed said uniformdistance from said intersection being disposed on the same side of saidline as said flrst row, all of the slots in said second row beingdisposed a common distance from said intersection, all of the slotsdisposed said common distance from said intersection being disposed onthe same side of said line as said second row, said common distance andsaid uniform distance differing from each other by an odd multiple ofone-half a wavelength of the radiated energy, all of the radiating slotson one side of said line having a distance from said intersectionditiering from the distance from said intersection of all the radiatingslots on the other side of said line, the long dimensions of each ofsaid slots being oblique to the radius extending between the geometriccenter of the slot and said intersection, a second conductive platespaced from said irst plate to form a waveguide symmetrical about saidradiation axis, means coupled to said waveguide for exciting a singlewave energy .mode therewithin that will excite said slots wherebyelectromagnetic energy will be radiated therefrom along said axis.

5. A radiator for radiating a beam of electromagnetic energy along aradiation axis, said radiator comprising a first conductive platedisposed normal to said axis and having a plurality of elongatedradiating slots therethrough, all of said slots forming a pattern thatis asymmetrical about a line extending diametrically across said `plateand intersecting said radiation axis, said pattern including a iirstgroup of slots and a second group of slots,

vall of the slots in said first 'group forming semicircular .rows thatare disposed entirely on one side of said line and concentric about theintersection of said line and said axis, all of the slots in said secondgroup forming semicircular rows that are disposed on the other side of'said line and are concentric about said intersection, all

Of the geometric centers of the slots in any given row being disposedat-a predetermined radius Vfrom said intersection, all ot the slotsdisposed at a given radius from said center being disposed on the sameside of said line, the longest dimensions of one-half of the slots ineach row intersecting the radius through the `geometric center of vtheslot at a posiive 45 degree angle, the longest dimensions of each of theremaining halt of the slots intersecting the radius through thegeometric center thereof at a negative 45 degree angle, the radius ofany given row differing from the radius of any given row in the othergroup by an odd integer multiple of one-half a wavelength of theradiated energy, and means coupled to said plate for inducing apredetermined current therein for exciting said slots whereby a beam ofelectromagnetic energy will be radiated along said axis.

6. A radiator for radiating a beam of electromagnetic energy along aradiation axis, said radiator comprising a iirst conductive platedisposed normal to said axis and having a plurality of radiatingapertures therethrough, all of the apertures through said plate forminga pattern thatis asymmetrical about a line extending ldiametricallyacross said plate and intersecting said axis, said pattern including afirst row of apertures and a second row of apertures, all of theapertures in said first row being disposed a uniform distance from saidintersection, all of the apertures disposed said uniform distance fromsaid intersection being disposed on one side of said line, all of theapertures in said second row being disposed a common distance from saidintersection, all of the apertures disposed said common distance fromsaid intersection disposed on the opposite side of said line, saidcommon distance and said uniform distance diiiering from each other bysome predetermined amount, all of the radiating apertures on one side ofsaid line having a distance from said intersection differing from thedistance from said intersection of all of the radiating apertures on theother side ot said line, a second `conductive plate having a conicalshape juxtaposed to said iirst plate for forming a conical waveguidesymmetrical about said radiation axis, and means coupled to saidwaveguide for exciting a single wave energy -mode therewithin that willexcite said apertures whereby a beam of electromagnetic energy wiil beradiated along said axis.

7. A radiator for radiating a beam of electromagnetic energy along aradiation axis, said lradiator comprising a rst conductive platedisposed normal to said axis and lhaving a plurality of elongatedradiating slots therethrough, all of said slots forming a pattern thatis asymmetrical about a line extending diametrically across said plateand intersecting said radiation axis, said pattern including a firstgroup of slots and a second group of slots, all of the slots in saidiirst group forming semicircular rows that are disposed entirely on oneside of said line and concentric about the intersection of said line andVsaid axis, all of the slots in said second group forming semicircularrows that are disposed on the other side of said line and are concentricabout said intersection, all of the geometric centers of the slots inany given row being disposed at a predetermined radius from saidintersection, all of the slots disposed at a ygiven radius from saidcenter being disposed on the same side of said line, the longestdimensions of one-half of the slots in each row intersecting the radiusthrough the geometric center of the slot at a positive 45 degree angle,the longest dimensions of each of the remaining half of the slotsintersecting the radius through the geometric center thereof atReferences Cited in the le of this patent UNITED STATES PATENTS RibletJuly 3l, 1951 2,562,332 y 2,756,421 Harvey July 24, 1956 2,838,754Bickrnore June 19, 1958 FOREIGN PATENTS ,i 1,014,859 Great Britain June'25, 1952 Non A lUNHED STATES PATENTOFTCE CERTIFlCATE OF CORRECTIONPatent No. 21929TO64 March l5.,L 1960 Kenneth C, Kelly It is herebycertified that error appears in the-printed specification of the abovenumbered patent requiring correction and that the said Letters Patentshould read as corrected below..

Column 7u line 9n strike out "from said common point `lfferng from thedistance".

Signed and sealed thle 23%@ dey of August-l960 (SEAL) Attest:

KARL H AXLINE ROBERT C. WATSON Attesting Officer Commissioner of Patents

